The invention relates to a method of processing X-ray images in which the difference is formed between two stored images, each of which has been formed from at least one X-ray image. The invention also relates to a device for the processing of X-ray images, comprising an X-ray image-forming device for producing X-ray images, at least two memories, each of which is used for the storage of an image which has been formed from at least one X-ray image, arithmetic means for determining the difference between the two images stored in the memories, and display means for displaying the differential image.
A method and a device of this kind are known from European Patent Publication No. 00 37.722. Such a device enables, for example, clear display of the cardiovascular system of the human body by determination of differential images on a pixel-by-pixel basis from X-ray images which are formed before as well as after the injection of a contrast medium into the cardiovascular system. The differential images should reveal only changes (the vascular system with/without medium). The part of the X-ray images which does not change for example, bone and tissue structures, should be completely eliminated by the determination of the differential image. In the ideal case, the differential image equals zero, except where the blood vessels containing contrast medium are present. The difference in contrast in a differential image is very small (a few percent), so that it is necessary to increase the contrast in order to achieve adequate visibility on, for example, a television screen. It is also necessary to add a so-called grey pedestal (constant value) to the signal values of the differential image in order to bring these signal values within the operating range of the television monitor.
It has been found, that the grey level fluctuates during the determination and display of the differential images on the television monitor. These fluctuations are due to the fact that the X-ray pulses and/or the signal transmission in the image intensifier/image pick-up tube network producing the X-ray images are not constant. Not only switch-on phenomena, but also variations of the X-rays (dip in the high voltage due to switching on of other hospital equipment) and, for example, fluctuations in the gain of the image intensifier have a serious adverse effect on the differential images to be displayed.
The use of video AGC circuits (automatic gain control circuits) does not offer an adequate solution to this problem in such devices. Circuits of this kind have a control time constant so that, for example, the first (two) X-ray images cannot be effectively used for the determination of the differential image. The foregoing holds good only when a switch-on phenomenon is concerned. If the X-rays exhibit further fluctuations, an AGC circuit imposes additional problems. This will be elaborated hereinafter.